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  • 1
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    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 44 (19). pp. 9957-9966.
    Publication Date: 2020-02-06
    Description: Proxy data suggest the onset of Northern Hemisphere glaciation during the Plio-Pleistocene transition from 3.2 to 2.5 Ma resulted in enhanced climate variability at the obliquity (41 kyr) frequency. Here, we investigate the influence of the expanding Greenland ice sheet (GrIS) on the mean climate and obliquity-related variability in a series of climate model simulations. These suggest that an expanding GrIS weakens the Atlantic Meridional Overturning Circulation (AMOC) by ~1 Sv, mainly due to reduced heat loss in the Greenland-Iceland-Norwegian Sea. Moreover, the growing GrIS amplifies the Hadley circulation response to obliquity forcing driving variations in freshwater export from the tropical Atlantic and in turn variations of the AMOC. The stronger AMOC response to obliquity forcing, by about a factor of two, results in a stronger global-mean near-surface temperature response. We conclude that the AMOC response to obliquity forcing is important to understand the enhanced climate variability at the obliquity frequency during the Plio-Pleistocene transition.
    Type: Article , PeerReviewed
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  • 2
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    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 46 (19). pp. 10865-10873.
    Publication Date: 2022-01-31
    Description: The variability of the Atlantic Meridional Overturning Circulation (AMOC) and its governing processes during the Last Glacial Maximum (LGM) is investigated in the Kiel Climate Model (KCM). Under LGM conditions, multidecadal AMOC variability is mainly forced by the surface heat flux variability linked to the East Atlantic pattern (EAP). In contrast, the multidecadal AMOC variability under preindustrial conditions is mainly driven by the surface heat flux variability associated with the North Atlantic Oscillation (NAO). Stand‐alone atmosphere model experiments show that relative to preindustrial conditions, the change in AMOC forcing under LGM conditions is tightly linked to the differences in topography. Key Points Multidecadal AMOC variability during the LGM and its associated physical processes have been investigated by means of a climate model Multidecadal AMOC variability during the LGM is mainly driven by surface heat flux variability linked to the East Atlantic pattern as opposed to the North Atlantic Oscillation under preindustrial conditions Change in topography during the LGM is responsible for the change in AMOC forcing
    Type: Article , PeerReviewed
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  • 3
    Publication Date: 2022-01-31
    Description: Quaternary East Asian winter monsoon (EAWM) evolution has long been attributed to high-latitude Northern Hemisphere climate change. However, it cannot explain the distinct relationships of the EAWM in the northern and southern East Asian marginal sea in paleoclimatic records. Here we present an EAWM record of the northern East China Sea over the past 300 ka and a transient climate simulation with the Kiel Climate Model through the Holocene. Both proxy record and simulation suggest anticorrelated long-term EAWM evolution between the northern East China Sea and the South China Sea. We suggest that this spatial discrepancy of EAWM can be interpreted as El Niño–Southern Oscillation (ENSO)-like controlling, which generates cyclonic/anticyclonic wind anomalies in the northern/southern East Asian marginal sea. This research explains much of the controversy in nonorbital scale variability of Quaternary EAWM records in the East Asian marginal sea and supports a potent role of tropical forcing in East Asian winter climate change.
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  • 4
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    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Oceans, 124 (5). pp. 3393-3410.
    Publication Date: 2022-01-31
    Description: Sea surface waves significantly affect the wind energy input to the Ekman layer in the upper ocean. In the study, we first incorporated the wave-induced Coriolis-Stokes forcing, the reduction of wind stress caused by wave generation, and wave dissipation into the classical Ekman model to investigate the kinetic energy balance in the wave-affected Ekman layer. Then, both the theoretical steady state solution for the idealized condition and the nonsteady state solution for the realistic ocean were derived. Total energy input to the wave-affected Ekman layer includes the wind stress energy input and the wave-induced energy input. Based on the WAVEWATCH III model, the wave spectrum was simulated to represent realistic random directional wave conditions. The wind stress energy input and the wave-induced energy input to the wave-affected Ekman layer in the Antarctic Circumpolar Current in the period from 1988 to 2010 were then calculated. The annual mean total energy input in the Antarctic Circumpolar Current region was 402.5 GW and the proportions of the wind stress energy input and the wave-induced energy input were, respectively, 85% and 15%. Particularly, total energy input in the Antarctic Circumpolar Current in the wave-affected Ekman layer model was 59.8% lower than that in the classical Ekman model. We conclude that surface waves play a significant role in the wind energy input to the Ekman layer.
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  • 5
    Publication Date: 2023-02-08
    Description: Stable oxygen isotope records from central Greenland suggest disproportionally large long‐term surface air temperature (SAT) variability during the Last Glacial Maximum (LGM) relative to preindustrial times. Large perturbations in mean atmospheric circulation and its variability forced by extensive Northern Hemisphere ice sheet coverage have been suggested as cause for the enhanced Greenland SAT variability. Here, we assess the factors driving Greenland SAT variability during the LGM by means of dedicated climate model simulations and find remote forcing from the Pacific of critical importance. Atmospheric teleconnections from the Interdecadal Pacific Oscillation (IPO), a multidecadal oscillation of sea surface temperature in the Pacific Ocean, strongly intensify under LGM conditions, driving enhanced surface wind variability over Greenland, which in turn amplifies SAT variability by anomalous atmospheric heat transport. A major role of the IPO in forcing Greenland SAT variability also is supported by a number of models from the Paleoclimate Modeling Intercomparison Project Phase III
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  • 6
    Publication Date: 2023-09-27
    Description: Climate models generally underestimate the pronounced warming in the sea surface temperature (SST) over the North Atlantic during the mid‐Pliocene that is suggested by proxy data. Here we investigate the influence of the North Atlantic cold SST bias, which is observed in many climate models, on the simulation of mid‐Pliocene surface climate in a series of simulations with the Kiel Climate Model. A surface freshwater‐flux correction is applied over the North Atlantic, which considerably improves simulation of North Atlantic Ocean circulation and SST under present‐day conditions. Using reconstructed mid‐Pliocene boundary conditions with closed Bering and Arctic Archipelago Straits, the corrected model depicts significantly reduced model‐proxy SST discrepancy in comparison to the uncorrected model. A key factor in reducing the discrepancy is the stronger and more sensitive Atlantic Meridional Overturning Circulation and poleward heat transport. We conclude that simulations of mid‐Pliocene surface climate over the North Atlantic can considerably benefit from alleviating model biases in this region.
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